<!DOCTYPE html>
<html>
<head>
  <meta charset="UTF-8">
  <title>Document</title>
</head>
<body>
  <script>
    var arr = [80, 99, 60, 57, 69, 71, 80, 77, 92, 90];
    var sum = 0;
    for (var i = 0; i < arr.length; i++) {
      sum += arr[i];
    }
    console.log(sum / arr.length);
  </script>  
</body>
</html>